Sound Reinforcement

Question 1

What is the speed of sound?

Answer 1

1130ft./second (at sea level and temp 59 degrees F

Question 2

What is the overall frequency band of the human voice?

Answer 2

100Hz - 6kHz

Question 3

Outdoor audio systems face three things to overcome. What are they and what is the effect of each?

Answer 3

Wind (velocity, gradient)
Temp (speed)
Humidity (attenuation)

Question 4

Each time you double the number of open microphones, the system gain must be what to avoid feedback?

Answer 4

System gain must be reduced by 3dB to avoid feedback per microphone

Question 5

What is the difference between a condenser microphone and a dynamic microphone?

Answer 5

Condensers use amplifiers; used in recording.

Dynamic - stage use, like a miniature loudspeaker.

Question 6

What can cause “hum” noise in a sound system?

Answer 6

Bad power supply / AC leakage.

Question 7

A loudspeaker’s sensitivity rating denotes what?

Answer 7

Efficiency rating of the driver

Question 8

What converts electrical energy into acoustic energy?

Answer 8

Output transducers

Question 9

What converts acoustic energy into electrical energy?

Answer 9

Input transducers

Question 10

Define dynamic range

Answer 10

The difference (in decibels) between the loudest and the quietest portion of the program.

Question 11

What is special about a lavalier microphone?

Answer 11

Doesn’t have proximity effect; attached to the person so sound quality is more consistent.

Question 12

A passive crossover is sensitive to two things. What are they?

Answer 12

Source impedance (driving), and load impedance

Question 13

What audio component can directly drive a loudspeaker?

Answer 13

No answer provided on sheet

Question 14

Define what each of the following does:
Multi-band equalizer - 
High pass filter - 
Low pass filter - 
Compressor - 
Limiter - 
Noise Gate -

Answer 14

Multi-band equalizer - allows the frequencies of each band to be adjusted.
High pass filter - (low cut filter) allows all frequencies above its cutoff point to pass through from filter input to filter output without attenuation.
Low pass filter - (high cut filter) attenuates frequencies above a certain cutoff point
Compressor - ratio of change from input level to an output level.
Limiter - allows signals below a specified input power or level to pass unaffected while attenuating (lowering) the peaks of stronger signals that exceed this threshold
Noise Gate - used to control the volume of an audio signal; attenuates signals that register below the threshold.

Question 15

Define what each of the following does:
Multi-band equalizer - 
High pass filter - 
Low pass filter - 
Compressor - 
Limiter - 
Noise Gate -

Answer 15

Multi-band equalizer - allows the frequencies of each band to be adjusted.
High pass filter - (low cut filter) allows all frequencies above its cutoff point to pass through from filter input to filter output without attenuation.
Low pass filter - (high cut filter) attenuates frequencies above a certain cutoff point
Compressor - ratio of change from input level to an output level.
Limiter - allows signals below a specified input power or level to pass unaffected while attenuating (lowering) the peaks of stronger signals that exceed this threshold
Noise Gate - used to control the volume of an audio signal; attenuates signals that register below the threshold.

Question 16

What are the three functional groups of an audio system?

Answer 16

input transducers, signal processing, and output tranducers

Question 17

Define compression ratio, as applied to audio systems.

Answer 17

An audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds thus reducing or compressing an audio signal’s dynamic range. The amount of gain reduction is determined by ratio: a ratio of 4:1 means that if input level is 4 dB over the threshold, the output signal level is 1 dB over the threshold. The gain (level) has been reduced by 3 dB:

Threshold = −10 dB
Input = −6 dB (4 dB above the threshold)
Output = −9 dB (1 dB above the threshold)

Question 18

What is the typical impedance of a balanced, low-impedance line input?

Answer 18

600 Ω

Question 19

What types of cables should typically be kept to 10 feet or less in length?

Answer 19

unbalanced cables

Question 20

What is the common name for a pin connector and who invented it?

Answer 20

RCA

Question 21

What is/are the primary cause(s) of noise in a sound system?

Answer 21

ground loop, grounding

Question 22

What are the best connections for rejecting ground loops and other electrical noise?

Answer 22

differential connections

Question 23

What piece of test equipment would you use in the following situations:

a. You need to test the polarity of a circuit.
b. You need a known signal source.
c. You have a pink noise generator as your test signal source.
d. When you want to record the physical waveform display.
e. You need to measure loudness.

Answer 23

a. You need to test the polarity of a circuit
b. You need a known signal source.
c. You have a pink noise generator as your test signal source.
d. When you want to record the physical waveform display.
e. You need to measure loudness.’‘The following items are used for basic measurement of voltages, currents, and components in the circuit under test.

Voltmeter (Measures voltage)
Ohmmeter (Measures resistance)
Ammeter, e.g. Galvanometer or Milliameter (Measures current)
Multimeter e.g., VOM (Volt-Ohm-Milliammeter) or DMM (Digital Multimeter) (Measures all of the above)
LCR meter - inductance (L), capacitance (C) and resistance (R) meter (measure LCR values)

The following are used for stimulus of the circuit under test:
Power supplies
Signal generator
Digital pattern generator
Pulse generator
Howard piA digital multimeter

The following analyze the response of the circuit under test:
Oscilloscope (Displays voltage as it changes over time)
Frequency counter (Measures frequency)

Question 24

Any piece of equipment used for sound reinforcement should cover what frequency range?

Answer 24

No answer provided on sheet

Question 25

What is the best shielding you can use in a permanent install, or within a rack or piece of equipment?

Answer 25

Foil - it’s cheaper and lightweight. Braided is more versatile for moving around

Question 26

What is the function of a mixer?

Answer 26

An audio mixer takes two or more audio signals, merges them together and provides one or more output signals.

Question 27

With respect to cabling, magnetic fields are only canceled by two things. What are they?

Answer 27

physical distance and twisting